In a typical networked system, a number of network devices, such as routers, are coupled to each other via a network (e.g., an optical fiber network). Data packets are sent between each other via the network according to a protocol adopted by the network (e.g., interior BGP).
FIG. 1A shows a conventional networked system. The system 1000 includes three routers 1010-1030. The routers 1010-1030 are coupled to each other with an interior BGP (iBGP) network 1100 running among them. The routers 1010-1030 may be referred to as BGP peers of each other. The system 1000 further includes a customer device 1200 coupled to router 1010 and router 1020 via a primary path A 1110 and a backup path B 1120, respectively.
Each of the routers 1010-1030 typically includes a routing and forwarding engine. The transmission of route information to the routing and forwarding engine is referred to as downloading and the transmission of route information from the routing and forwarding engine is referred to as redistribution in this document.
In general, a routing protocol downloads active route entries to a RIB stored in the routing and forwarding engine, which in turn selects a route entry to be forwarded to other network devices in the iBGP network 1100. To select the best route on the routers, some or all of LOCAL_PREF, AS_PATH, MED, and IGP metric of the active route entries are compared. Based on some predetermined criteria, one of the active route entries is selected. The selected route entry is downloaded to the routing and forwarding engine and advertised to other BGP peers in the networked system. It is common to redistribute route entries from other routing protocols, such as Open Shortest Path First (OSPF), Routing Information Protocol (RIP), and static routing, etc., into BGP in order to implement various routing policies. A redistributed route typically has an empty AS_PATH attribute and a zero IGP metric.
However, the interaction of redistributed routes and other BGP routes may be order-dependent, and the BGP route selected can thus be non-deterministic. Consequently, complicated configurations are sometimes used to deploy simple routing requirements (such as primary and backup connections between a network of routers and a customer device).
One common routing setup for a multi-homed customer is to treat one connection as the primary, and another connection as the backup. Consider the example illustrated in FIG. 1, where the primary path is path A 1110 and the backup path is path B 1120. The customer device 1200 is statically routed on the router 1010 and the router 1020, and is redistributed into the iBGP network 1100. On the router 1020, the backup path B 1120 for the customer device 1200 is configured with a less preferred administrative distance than the primary path A 1110.
To get to the router 1020, there are two route entries of two potential BGP paths for the customer device 1200, which include a locally redistributed path associated with the backup path B 1120 and an iBGP path learned from the router 1010 (i.e., the primary path A 1110 via the router 1010). Depending on the order of arrival of the two route entries corresponding to these two paths, the routing behavior of the router 1020 may differ. When the iBGP path from the router 1010 is downloaded to the router 1020 first, the iBGP path would be selected, and then downloaded to the RIB of the router 1020. Due to the more preferred value of the administrative distance, the routing and forwarding engine may select the iBGP path, and thus, the local path B 1120 would serve as a backup and would not be redistributed (assuming that only the active path is redistributed). As a result, the network devices in the network 1100 would converge to the primary path A 1110 on the router 1010.
However, if the iBGP route entry from the router 1010 is downloaded to the router 1020 later than the locally redistributed route entry is, then the two paths are compared for route selection. By default, LOCAL_PREF, AS_PATH, and MED are the same in some conventional routers. However, since the IGP metric of the locally redistributed route entry is usually more favored than the IGP metric of the route entry of the iBGP path, the locally redistributed route entry would be selected to be redistributed to other iBGP peers in the network 1100. As a result, the intended backup path B 1120 is selected as the primary path on the router 1020. Furthermore, the router 1020 may advertise the selected route entry to the rest of the network 1100, and hence, some portions of the network (such as the router 1030) may converge to use the backup path B 1120 from the router 1020 as well.
An existing approach to eliminate the non-deterministic routing behavior of the router 1020 is to configure lower LOCAL_PREF for the redistributed route entries. Some existing approaches further modify any other vendor specific route selection criteria preceding the LOCAL_PREF comparison. However, this approach may increase the operational complexity and cost.